Fishermen in the Philippines accidentally caught a megamouth shark, one of the rarest fish in the world and later ate it after it had been identified. Only 40 other sightings of the shark have ever been recorded, according to the World Wildlife Fund.

The 500kg, four metre megamouth (Megachasma pelagios) died while struggling in the fishermen's net on 30 March off Burias island in the central Philippines. It was taken to nearby Donsol in Sorsogon province, said Gregg Yan, a spokesman for WWF-Philippines.

WWF project manager, Elson Aca, took pictures of the megamouth and tried to dissuade the fishermen from eating it. However, they went against his advice and made a local speciality dish called kinunot - shark meat sauteed in coconut milk.

The first megamouth was discovered in Hawaii in 1976, prompting scientists to create a new family and genus of sharks. The discovery was hailed as the marine find of the 20th century, rivalling the discovery of the coelacanth in the 1930s.

The megamouths are docile filter-feeders. Their blubbery mouths are almost one metre wide and are lined with a brilliant silver band to attract plankton. Yan said the Burias megamouth's stomach revealed it had been feeding on shrimp larvae.

Yan said the fish was tagged Megamouth 41, the 41st megamouth recorded in the world by the Florida museum of natural history. It was the eighth reported megamouth encountered in Philippine seas.

He said the megamouth was caught in waters 200 metres in depth, which are also frequented by the endangered whale shark, the world's largest fish and also a filter-feeder in the Donsol area, about 185 miles south-east of Manila.

"The presence of two of the world's three filter-feeding sharks warrants special attention for the Donsol-Masbate region," said Aca. "Whale and megamouth sharks, manta rays, dolphins and other charismatic giants indicate that the region's ecosystem is still relatively healthy. By protecting megafauna, we help maintain the dynamic balance of our seas and ensure the entire ecosystem's resilience and natural productivity."

Yan urged fishermen who encounter the rare shark to immediately report to the authorities or the WWF.